A
paranoid temperament is in the genesat least when it comes to plants.
This is the lesson from studying a dwarf mutant of Arabidopsis thaliana,
or mouse-ear cress, which behaves as if it were under constant attack
from microbes. Investigating this mutant, a team of Danish researchers
and their American and English collaborators have pinpointed a gene with
a crucial role in regulating natural resistance against disease. Their
findings open a view into the enigmatic immune system of plants and point
a way to creating more robust crops.

Wildtype Arabidopsis and the MPK4 knock out. Courtesy John Mundy

"Globally, there is obviously significant interest in protecting crops
against disease. And a deeper understanding of the genetic basis for resistance
will be instrumental in developing and breeding better plants," says plant
molecular biologist John Mundy of Copenhagen University's Institute of
Molecular Biology who led the Arabidopsis study. Recently published
in Cell, the team's major finding is the identification of the
gene map kinase 4 (MPK4) as a regulator of resistance to a range of pathogens
including bacteria, viruses or fungi. MPK4 controls what is known as systemic
acquired resistance, or SAR, which occurs as a response to invasion by
microbes. During SAR, plants react by producing and circulating salicylic
acid, which acts as an internal signal to trigger expression of a range
of anti-microbial proteins. The result is broad and long-lasting immunity.
It now turns out that the activity of MPK4 is normally required to suppress
SAR, indicating that MPK4 activity must be turned off in the face of attack
to allow the development of SAR.

"It is very surprising that the 'inactivation' of a map kinase is used to
turn on a plant resistance mechanism," says plant biochemist Dierk Scheel
of the Institute of Plant Biochemistry in Halle, Germany. And according
to plant geneticist Jérôme Giraudat of the Institute of Plant Sciences in
Gif-sur-Yvette, France, "this counter intuitive observation adds important
new knowledge of protein kinase signaling." While the new study is a major
step forward for basic science, molecular biologist Morten Petersen of Copenhagen
University stresses the significant potential for practical application.
"Our findings could allow geneticists and plant breeders to purposefully
push the right buttons for activating the natural defense mechanisms and
turning on plant resistance," he says.

It is very surprising that the 'inactivation' of a map kinase is used
to turn on a plant resistance mechanism

Applying the new knowledge of MPK4 could take several directions. "One
obvious route is to create transgenic plants in which MPK4 is permanently
shut off just as in the paranoid Arabidopsis mutant," says Giraudat.
Such inherently resistant plants would not only hold the promise of decreasing
agriculture's pesticide load but could prove especially attractive for
poor farmers in developing countries who do not have access to pesticides.
Peter Brodersen of Copenhagen University points to another interesting
avenue, which would appeal to consumers who are wary of genetically manipulated
crops and food. "If we could identify harmless chemical compounds able
to transiently turn off MPK4 activity, they could be used to spray crops
and make them resistant at any time they face a microbial attack."

Originally, the team didn't set out to elucidate the intricacies of plant
immunity. Rather, their project is a perfect illustration of how the broad
approach of functional genomics, which generally aims at assigning biological
function to genetic sequences, can lead anywhere. "One very effective
way to identify genes that play essential roles in the normal organism
is to knock them out. Then you can get at their function by analyzing
the biological effects on the mutant," explains Mundy. His group enlisted
nature's own genomic vandalizersknown as transposonsfor creating
a series of Arabidopsis genetic knock outs. These small, mobile
and virus-like DNA elements have been a part of plant genomes for millions
of years and most of the time they lie dormant and do no harm. However,
under certain natural or experimental conditions they become active in
the replicating DNA of forming seeds. This way they wreak havoc in the
genome of the next generation by randomly inserting themselves.

"When you observe a phenotype that differs markedly from the wild type,
you have hit a functionally important gene," explains Petersen who was
immediately alerted by the dwarf Arabidopsis. Subsequent genetic
analysis revealed that a transposon had disrupted the MPK4 gene, preventing
it from being transcribed. Using a specialized DNA array containing thousands
of Arabidopsis genes (an EST microarray), the researchers then
analyzed gene expression in their mutant and quickly found that MPK4 somehow
controls the expression of anti-microbial genes of the SAR response. "This
underscores the power of having access to genetic sequences," says Scheel.
He and others in the field applaud that Arabidopsis, which serves
as the lab rat and model of choice in plant science, has recently become
the first plant to have its genome fully sequenced.

While the Arabidopsis sequence may speed up plant science in general,
the new MPK4 study will play into the hands of researchers interested
in map kinase signaling, says Giraudat. "By providing the first-ever plant
map kinase mutant, it really opens up possibilities for genetic studies
of the signaling pathways." And certainly, the interest in map kinases
is exploding. These proteins are found on every branch of the evolutionary
tree from yeast to humans, and they play crucial roles in controlling
the delivery of molecular messages inside cells. The insight into map
kinase function in plants is lagging behind, but Scheel predicts it will
be of major importance. "Plants have a large number of map kinase cascades
suggesting that their signaling networks are very complex," he says. Map
kinases are involved in different signaling chains responding to a wide
range of environmental stimuli and stresses, and a detailed understanding
of the cross talk between them is a prerequisite for accurate manipulation
of plant traits. Adds Mundy, "clearly, we must know exactly which processes
we affect when we start tinkering with living systems."